Ireland (Éire in Irish) is the third-largest island in Europe. It lies on the west side of the Irish Sea, and is part of the geographical (but not political) British Islands archipelago. It is composed of the Republic of Ireland which covers five sixths of the island (south, east, west and north-west) and Northern Ireland, currently a part of the United Kingdom, which covers the northeastern sixth of the island.

The Iron Age in Ireland is associated with people now known as Celts. These are traditionally thought to have colonised Ireland in a series of waves between the 8th and 1st centuries BC, with the Gael, the last wave of Celts, conquering the island and dividing it into five or more kingdoms. Many scholars, however, now favour a view that emphasises cultural diffusion from overseas over significant colonisation. The Romans referred to Ireland as Hibernia. Ptolemy in AD 100 records Ireland's geography and tribes. Native accounts are confined to Irish poetry, myth, and archaeology. The exact relationship between Rome and the tribes of Hibernia is unclear; the only references are a few Roman writings.

Tradition maintains that in AD 432, St. Patrick arrived on the island and, in the years that followed, worked to convert the Irish to Christianity. The druid tradition collapsed in the face of the spread of the new faith. Irish scholars excelled in the study of Latin learning and Christian theology in the monasteries that flourished, preserving Latin learning during the Dark Ages. The arts of manuscript illumination, metalworking, and sculpture flourished and produced such treasures as the Book of Kells, ornate jewelry, and the many carved stone crosses that dot the island.

In 1172, King Henry II of England gained Irish lands, and from the 13th century, English law began to be introduced. English rule was largely limited to the area around Dublin known as the Pale, but this began to expand in the 16th century with the final collapse of the Gaelic social and political superstructure at the end of the 17th century, as a result of the English and Scottish Plantation of Ulster and other plantations in Leix ("King's County", modern day Laois) and Offaly ("Queen's County"). In 1801 the unrepresentative Irish parliament was coerced and bribed to vote itself out of existence and for a union with the Kingdom of Great Britain (itself a union of England and Scotland, created almost 100 years earlier), to create the United Kingdom of Great Britain and Ireland.

The debate over home rule led to tensions between Irish nationalists and Irish unionists (those who favoured maintenance of the union). Most of the island was predominantly nationalist, Catholic and agrarian. The northeast, however, was predominantly unionist, Protestant and industrialised. Unionists feared a loss of political power and economic wealth in a predominantly nationalist, Catholic home rule state. Nationalists believed that they would remain economically and politicially second class citizens without self-government.

An attempt was made to gain independence for Ireland with the 1916 Easter Rising, an insurrection largely confined to Dublin. Though support for the insurgents was small, the violence used in its suppression (being considered a serious treason in time of war) led to a swing in support of the rebels. The unprecedented threat of Irishmen being conscripted to the British Army for service in France accelerated this change. In December 1918 most voters voted for Sinn Féin, the party of the rebels. Having won three-quarters of all the seats in Ireland, its MPs assembled in Dublin on 21 January 1919 to form an Irish parliament, Dáil Éireann. A war of independence often called the Anglo-Irish War raged from 1919 to 1921. In mid-1921 the Irish and British governments signed a truce that halted the war. In late 1921 an Anglo-Irish Treaty was signed between representatives of both governments. This created an Irish self-governing dominion called the Irish Free State. Under the Treaty Northern Ireland could opt out of the Free State and stay with the United Kingdom. This was a foregone conclusion and Northern Ireland promptly did so. For most of the next 75 years, each territory was strongly aligned to either Roman Catholic or Protestant ideologies, although this was more marked in the six counties.

The new Irish Free State (1922–37) existed against the backdrop of the growth of dictatorships in Europe and a major world economic downturn. In contrast with many contemporary European states it remained a democracy, in which the losing faction in the Irish civil war, Eamon de Valera, was able to take power by winning the 1932 general election. In contrast to many other states in the period, the Free State remained financially solvent. However, unemployment and emigration were high.

In 1937, a new Constitution of Ireland proclaimed the state of Éire (or Ireland). The state remained neutral throughout World War II and this saved it from the horrors of the war, although tens of thousands volunteered to serve in the British forces. Ireland was also hit badly by rationing of food, and coal in particular (peat production became a priority during this time). Though nominally neutral, recent studies have suggested a far greater level involvement by the South with the Allies than was realised, with D Day's date set on the basis of secret weather information on Atlantic storms supplied by the Republic. In 1949 the state was formally declared the Republic of Ireland and it left the British Commonwealth.

Economic downturn in the 1970s, augmented by a set of misjudged economic policies followed by Taoiseach Jack Lynch, caused the Irish economy to stagnate. However, economic reforms in the late 1980s and considerable investment from the European Community led to the emergence of one of the world's highest economic growth rates, with mass immigration (particularly of people from Asia and Eastern Europe) as a feature of the late 1990s. This period came to be known as the Celtic Tiger and was focused on as a model for economic development in the former Eastern Bloc states, which entered the European Union in the early 2000s.

From 1921 to 1971, Northern Ireland was run by the Ulster Unionist Party government, based at Stormont in East Belfast. The founding Prime Minister, James Craig, proudly declared that it would be "a Protestant State for a Protestant People" (in contrast to the "Papist" state to the south). Discrimination against the minority nationalist community, and their total exclusion from political power (gerrymandering), led to the appearance of a civil rights campaign in the late 1960s. A violent counter-reaction from the Royal Ulster Constabulary (RUC) and right-wing unionists such as the Rev. Ian Paisley led to civil strife. Tensions came to a head with the events of Bloody Sunday and Bloody Friday, and the worst years (early 1970s) of what became known as The Troubles resulted. The Stormont majoritarian government was prorogued in 1971 and abolished totally in 1972. Paramilitaries such as the traditional republican Provisional IRA, and the Marxist Official IRA, unionist groups like the Ulster Defence Association and the Ulster Volunteer Force, and the British army and the RUC fought a bitter "war", which resulted in the deaths of thousands of men, women and children, civilians and military. Most of the violence took place in the six counties, but some also spread to England and across the border.

More recently, the Good Friday Agreement of April 10, 1998 has brought a degree of power sharing to Northern Ireland, giving both unionists, who favour it remaining a part of the United Kingdom, and nationalists, who favour it becoming part of an All-Ireland state (not necessarily the Republic of Ireland), control of limited areas of government. However, both the power-sharing Executive and the elected Assembly have been suspended since October 2002 following a breakdown in trust between the political parties. Efforts to resolve outstanding issues, including "decommissioning" of paramilitary weapons, policing reform and the removal of controversial British army bases are continuing.